1. Field of the Invention
The present invention relates to an apparatus which is used for collecting and separating a two-phase fluid and dispensing a light phase.
2. Description of the Prior Art
It is well known in the art to collect and centrifuge fluids, such as blood, in small bore or capillary tubes for clinical evaluation. It has been possible to collect blood specimens from skin punctures directly into narrow bore capillary tubes. The collection tube may then be placed in a centrifuge for separating the fluid into a light phase of serum or plasma and a heavy phase of blood cells. The separated serum is then analyzed. An anticoagulant may be added to the blood specimen before centrifugation if testing is to be performed on plasma.
Most tests are conducted on blood serum or blood plasma after it has been removed from the collection vessel. The tests may be performed on automated chemistry and immunological analyzers which require specified volumes of blood plasma for the diverse tests. These tests are generally used to determine the physical state and health of the individuals being tested. Hence, attainment of the highest precision and accuracy are essential requirements in blood analyses.
There are several means by which the plasma or serum is conventionally obtained from the collection tube. This is frequently accomplished by decanting, siphoning, or the use of a pipet or automated vacuum withdrawal cannula. One major problem encountered with respect to such testing is that as the serum or plasma is removed from the collection tube after centrifugation, it is often inadvertently mixed with the heavy phase thereby making the test result inaccurate. Thus, testing should be done for reliable results shortly after separation. If blood is centrifuged and the separated cells are left in contact with the liquid phase, glucose will move from the serum and be metabolized by the cells. The cation potassium leaks out of the cells with the passage of time and enters into the plasma. Other analyses may have been found to change with time. Consequently, the reliability of the analysis may diminish if timely analysis is not made on centrifuged blood.
Several means of obtaining and maintaining a better separation of blood fluid into a light phase and a heavy phase are known in the art. In the system illustrated in my U.S. Pat. No. 3,508,653, a resilient plunger is positioned within a tubular container closely adjacent a resilient stopper. The resilient plunger which is in sealing contact with the inner surface of the tubular container moves to the interface between the plasma and the cells on centrifugation, providing a permanent seal. However, in dispensing the light phase, in order to get a specific volume of sample it is necessary to use an additional device such as a pipet or a pipetter-dispenser to transfer the sample to the test vessel. If a gel is used as an equivalent separator element, use of a pipet or pipetter-dispenser is likely to cause greater contamination and inaccuracy than decantation from the collection tube directly to the test vessel.
In the system of my U.S. Pat. No. 5,065,768, a gel separator element may be placed within a capillary tube having a self-sealing plug at one end opposite a fluid entry end. The separator gel forms a seal between the light phase and the heavy phase of the blood on centrifugation. A plunger of a pipetter-dispenser pushes the self-sealing plug to dispense a measured quantity of the light phase sample. However, the gels employed have a tendency to adhere to the wall of the tube resisting movement and the cells usually thereby diffuse and contaminate the light phase sample.
In U.S. Pat. No. 5,203,825, Haynes et al. disclose a collection tube for examining the buffy coat layer. A plastic float is placed within a capillary tube having a vented cap at one end. On centrifugation, the float is positioned between the light phase and the heavy phase, after which the buffy coat layer is examined. However, this tube is not intended or designed to dispense the light phase. Indeed, the cap's flange base required to seal its vent obviates use of a pipetter-dispenser.